Researchers at the University of Oxford have used a femtosecond laser to write hundreds of waveguides in sapphire, suggesting that sapphire photonic chips hold the promise of real-world feasibility.
Figure 1: 4cm long sapphire integrated photonic chip.
Photonic integrated circuits (PICs) require compact integration of photonic devices on a body substrate material, which is currently used mainly as glass. Glass has its own limitations, so researchers at the University of Oxford in the UK are exploring the use of sapphire as an alternative substrate material to glass.
Building high-quality integrated photonics circuits in sapphire could open up many new possibilities for applications such as communications, sensing or quantum computing.
"The basic building blocks of any compact photonic circuit are waveguides," says Mohan Wang, a researcher in the Department of Engineering Sciences at the University of Oxford, "and we can use laser fabrication to 'write' in sapphire in a designed pattern of ' waveguide arrays. When we inject light into the waveguide arrays, the light propagates along the designed path so that we can write hundreds of waveguides in sapphire for very complex functions."
Femtosecond laser writes waveguides in sapphire
Femtosecond lasers can write these waveguides into large chunks of material because femtosecond lasers are extremely intense and can be focused down to the micrometer scale. "This leads to nonlinear ionization within the material in the focal volume, which results in a change in refractive index." Wang said, "By relative motion between the femtosecond laser and the sapphire bulk material, which is mounted on a three-dimensional nanoprecision platform, along the designed trajectory, it is possible to write the integrated photonics paths that we designed on the sapphire substrate."
Waveguides are formed by regions of material with a high refractive index relative to their surrounding regions, and the most common material used in integrated photonics is glass.
"Exposing glass to a femtosecond laser increases its refractive index, so writing the waveguide by scanning the laser along the inside of the sample is simple." Wang says, "But in sapphire crystals, the laser decreases the refractive index. So instead of writing the waveguide where we want it, we write on the outside of it to lower the refractive index in the surrounding area. This is called a recessed cladding waveguide, and we used it in our previous work on sapphire fibers."
This time they have improved the process and reduced the optical loss of the waveguide compared to the group's previous work on sapphire. This allows them to now write waveguides that are 4cm long, which also means they can write more complex structures such as 1:2 optical splitters (see Figure 1).
The team optimized their waveguide building blocks and made multiple copies of them. "The process was very well controlled and all the results were the same. This made us realize that integrated sapphire photonic chips have a realistic prospect of feasibility."
Calibrating the laser writing process
A huge challenge in the process, however, was calibrating the laser writing process.
Wang explains that changes in refractive index "are critical for designing optimized structures, and this is especially true for crystals because they have a high refractive index and many refractive index measurements are destructive. But writing photonic circuits requires very precise control of the laser-modified profile, so rapid characterization is also desirable."
To do this quickly, the researchers wrote a linear array design to provide a unique output pattern. The pattern is directly correlated with changes in refractive index and can be used as a fingerprint, says Wang: "By correlating these patterns with a set of simulations, we can identify exponential modulation. It enables fast, reliable calibration before each fabrication."
Julian Fells, the lead researcher on the project, says that because sapphire is a very hard and resilient material, "it can withstand ultra-high temperatures of up to 2,000°C and high radiation. These properties make it suitable for extreme environments such as aerospace, space and power generation. In addition, sapphire has a very broad spectral window in the mid-infrared region, a window that can be used for medical applications. By increasing the complexity of photonic circuits, higher performance sensors and devices are expected."
The team has already demonstrated the basic building blocks of the photonic chip, and now they are actively working to reduce losses and extend the complexity of the circuits even further.
